Rapid response method for the failure of links between different routing domains

ABSTRACT

The invention relates to a rapid response method for the failure of a link between two routing domains in a packet-oriented network. Once the failure of a link has been identified, substitute routes are provided for the interrupted routes by the local selection of alternative routes and by the propagation of messages along the substitute routes. In contrast to conventional inter-domain protocols such as the BGP (Iborder gateway protocol) the transmission of messages and the associated modification to the routing only involves routing domains that lie along the replacement routes. In one embodiment, a network-wide propagation of messages takes place if the failure of the link represents a persistent breakdown. As a consequence, optimal routes are re-determined in the entire network. The invention provides breakdown compensation that is appropriate for temporary breakdowns and prevents instabilities that occur as a result of the use of conventional inter-domain protocols.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US National Stage of International ApplicationNo. PCT/EP2004/001171, filed Feb. 9, 2004 and claims the benefitthereof. The International Application claims the benefits of Europeanapplication No. 03004532.2, filed Feb. 28, 2003, both applications areincorporated by reference herein in their entirety.

FIELD OF INVENTION

The invention relates to a rapid response method for the failure of alink between two routing domains in a packet-oriented network.

SUMMARY OF THE INVENTION

The invention lies within the field of Internet technologies or, morespecifically, in the field of routing methods in packet-orientednetworks, and is directed at the transmission of data under real-timeconditions.

Currently, by far the most important development in the field ofnetworks is the convergence between voice and data networks. Animportant future scenario is that data, speech and video data will betransmitted over a packet-oriented network, whereby newly-developednetwork technologies ensure adherence to the requirements characteristicof the various classes of traffic. Future networks for the various typesof traffic will work on a packet-oriented basis. Current developmentactivities relate to the transmission of speech data over networksconventionally used for data traffic, primarily IP (Internet Protocol)based networks.

In order to permit voice communication over packet networks, and inparticular IP-based networks, with a quality which corresponds to thatof speech transmission over circuit-switched networks, such qualityparameters as for example the delay of data packets or the jitter mustbe kept within narrow limits. In the case of speech transmission, it isof major importance for the quality of the service offered that thedelay times do not significantly exceed values of 150 milliseconds. Inorder to achieve a correspondingly small delay, work is in progress onimproved routers and routing algorithms, which are intended to permitfaster processing of the data packets.

In the case of routing through IP networks, a distinction is usuallymade between intra-domain and inter-domain routing. For a datatransmission over the Internet, it is common for networks—one speakshere of subnetworks, domains or so-called autonomous systems—fromseveral network operators to be involved. The network operators areresponsible for the routing within the domains which fall into theirarea of responsibility. Within these domains they have the freedom toadapt the approach to routing as they wish, provided only that thequality of service characteristics can be adhered to. A differentsituation is represented by routing between various domains in whichvarious domain operators establish links with each other. Inter-domainrouting is made more complicated by the fact, on the one hand, that thepaths determined through the various domains to the destination shouldbe the most optimal possible, but on the other hand the domain operatorscan use local strategies, which makes it more difficult to calculatepaths which are globally optimal, according to objective criteria. Forexample, one strategy is to avoid the domains of network operators in aparticular country for traffic from a particular source. However, thisstrategy is not in general known to all the network operators withdomains through which the traffic is routed, i.e. a network operatormust make a local decision about the domain to which he forwardstraffic, without full information being available about the optimal pathas determined by some metric. The strategies are also often referred toby the English term “policies”.

For routing between various domains, use is made of so-called exteriorgateway protocols, EGPs. In the Internet, the Border Gateway ProtocolVersion 4 (border gateway protocol is often abbreviated to BGP), asdescribed in more detail in RFC (Request For Comments) 1771, iscurrently more often than not used. The border gateway protocol is aso-called path-vector protocol. A BGP instance (the term “BGP speaker”appears frequently in English-language literature) is informed by itsBGP neighbors about possible routes to the destinations which can bereached via the BGP neighbor concerned. Using path attributes, detailsof which are supplied at the same time, the BGP instance contains forthe reachable destinations what are, from its local point of view, theoptimal routes in each case. In the context of the BGP protocol, fourtypes of messages are exchanged between BGP instances, including aso-called update message with which route data is propagated throughoutthe entire network, and which permits the network to be optimized fortopology changes. The emission of update messages usually leads tomodification of the path data in all the network's BGP instances, forthe purpose of optimizing the routings according to the data availablelocally. Apart from this a part is played by so-called keep-alive orstatus confirmation messages, by which a BGP instance informs its BGPneighbors about its operability. In the absence of these messages, theBGP neighbors make the assumption that the link to the BGP instance isdisrupted.

The propagation of topology data by means of the BGP protocol has thedisadvantage that, when there are frequent change reports, a substantialload occurs from the messages propagated through the network to reportthe changes, and that the network does not converge to a new state ifthe change messages follow one another too rapidly. This problem, thatthe network does not converge to a new state, or that the inter-domainrouting does not become stable, has been tackled by the so-calledroute-flap damping approach. The idea of this concept is to apply asanction to the report of a change made by a BGP neighbor. On receipt ofa change message, the damping parameter is increased and, if the dampingparameter exceeds a threshold, change messages are ignored. In theabsence of change notices, the value of the damping parameter isreduced. As a consequence of this, change notices from BGP instances areignored if the frequency of the change notices is too high. The responseof the network to temporary instabilities is thereby slowed ordiminished. However, the method has the disadvantage that the responseof the network to malfunctions which are more long-lasting (theexpression “persistent errors” is used here) is delayed. In relation toreal-time traffic, above all, it is disadvantageous if malfunctions inthe Internet routing are only eliminated over a longer timescale.

The object of the invention is to specify a method which permits a rapidresponse to malfunctions during inter-domain routing and, at the sametime, avoids the disadvantages of conventional methods.

This object is achieved by the claims.

In accordance with the invention, when a link between two routingdomains fails, a substitute route or substitute path, as applicable, isprovided. The inter-domain routing along this substitute route is set upin such a way that data packets which would normally be routed via thedisrupted link are diverted along the substitute route to theirdestination. Here, ‘the term link failure refers to any malfunctionwhich interrupts the connection or connectivity between two routingdomains.

A routing domain (the expressions “autonomous system” or “subnetwork”are also found in the literature) is characterized by unified routingwithin the domain. For example, within a domain packets may be routedusing the OSPF (open shortest path first) protocol. Here, we present ameasure for routing between domains—referred to below as inter-domainrouting—which permits a rapid response to failures on links betweendomains. In this situation, the link failure is detected by one routingdomain. This could be, for example, by a router in the routing domain,which is equipped with protocol software for inter-domain routing. Inthe case of the BGP protocol we speak in such a case of a BGP speaker ora BGP instance. After the failure has been detected, the message aboutthe link failure is propagated, but not throughout the entire networkbut only along one or more substitute routes. Routers along thesubstitute route(s) adjust their inter-domain routing in such a way thatpackets can be routed along the substitute route(s). This is effected,for example, by changes to the routing tables for those routersbelonging to the domains along the substitute route which haveinter-domain protocol functionality. Further propagation of a messageabout the link failure along the substitute route by an EGP routinginstance can be omitted if the EGP routing instance already provides arouting, to the destination of the substitute route, which avoids thelink which has failed. This can arise as a result of the topology of thenetwork or alternatively due to the EGP routing instance having alreadyhad a notification from the opposite direction, e.g. originating fromanother EGP instance which has been informed of the link failure.

Routers with inter-domain protocol functionality are also referred tobelow as EGP instances. EGP (Exterior Gateway Protocol) is in this casea generic expression for inter-domain protocols such as, for example,the BGP protocol. The setting of the inter-domain routing for a domainon the substitute route can be effected in the following way: an EGPinstance receives a notification of the link failure. At this point, theEGP instance selects an alternative route for a route which passes viathe link which has failed. The EGP instance provides alternative routes,for example, from update messages in the BGP protocol, which have beenpropagated in the network and used by the EGP instance for the purposeof determining multiple routes to different destinations. The nextrouting domain on the alternative route can be identified by referenceto the alternative route. The address of a router—as a rule an EGPinstance—can then be specified as the next destination or next hop, asapplicable, for the routing in the EGP instance's routing table.

Here and in what follows, the following distinction is made between asubstitute route and an alternative route. The substitute route is theroute which is specified, on the basis of the method in accordance withthe invention, for routing via a route which includes the link which hasfailed. The alternative route refers to a local choice of route as analternative to the route which has failed. With the method, data canpropagated about not only the link which has failed, but also about theselected alternative route. However, in the preferred variant, this isnot done but instead an alternative route is always selected by therouting domains lying on the substitute route, on the basis of the dataabout the link failure. The reason for this is that different strategies(policies) are often applied by the individual network operators, thesebeing generally not known to the other network operators. Hence, it isoften only possible for a routing domain to make a decision which islocally valid, i.e. in relation to the routing, a decision about thenext destination along an alternative route. An alternative route whichis locally selected can, but does not necessarily have to, coincide withthe substitute route which is ultimately produced. It is thus possiblefor a routing domain to make a choice of alternative route which doesnot coincide with the choice by the routing domains which come beforethe substitute route.

The invention has the advantage that it is possible to react rapidly toa malfunction, without it being necessary to effect the propagation ofmessages throughout the entire network followed by convergence on a newstate in terms of the topology. In particular, for malfunctions whichare of limited duration, no resource-intensive fault response need takeplace.

Under a development, the routing domains which are first informed of themalfunction, or the routing domains which are, for example, at the twoends of a disrupted link, initiate the provision of substitute routesfor all the routes which pass along the disrupted link. For a disruptedroute, it is also possible to provide two or more substitute routes,where the additional substitute routes which are provided can be used asa backup or for the implementation of policies.

In a further development, provision is made for identifying more thanone alternative path. The additional alternative path(s) can then beused as a substitute for the preferred path or for a routing whichdepends on the routing strategies. For example, a decision may be madeabout which alternative path should be used by reference to data in thepacket header.

In a variant, a protocol is specified, to provide for a network-widepropagation of messages for the calculation of optimal routes, forexample the BGP protocol. Under this variant, when a link fails theredetermination of optimal routes for the inter-domain routing, to takeinto account the link failure, is suppressed for a period of time bymeans of the protocol. In the case of the BGP protocol, for example, theBGP process, on the router to which the link malfunction has beenreported, is restrained from sending update messages to other BGPinstances. In addition, the router which is neighboring to the linkmalfunction can act as a proxy for EGP instances which can no longer bereached, and can send keep-alive messages to mimic the orderlyfunctioning of the link which has failed to the BGP processes of otherBGP instances. This function, of suppressing messages, can be disabledafter the period of time, so that a second propagation of messages takesplace for the purpose of determining optimal routes. This developmenthas the advantage that it is possible to distinguish between short-termand longer lasting (“persistent”) malfunctions, whereby the response toshort-term malfunctions is the provision of substitute routes and forlonger-term malfunctions an appropriate modification of the topologywithin the overall network is initiated.

For the handling of short-term malfunctions, it is helpful in additionto identify the route which is replaced by the substitute route, in sucha way that it is ready for use again when a message is received aboutthe restoration of the link.

The subject matter of the invention is explained in more detail below inthe context of an exemplary embodiment, by reference to figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a conventional response when a link fails, using BGPinter-domain rerouting,

FIG. 2 shows a response in accordance with the invention to the failureof a link,

FIG. 3 shows a flow diagram of the protocol, and

FIG. 4 shows a use of routing arrays for the purpose of taking intoaccount routing strategies.

DETAILED DESCRIPTION OF INVENTION

FIG. 1 shows eleven autonomous systems or routing domains AS-1 to AS-11together with links which connect these autonomous systems to eachother. The autonomous systems communicate with each other with the helpof the BGP protocol, whereby individual routers in the autonomoussystems are equipped with appropriate protocol capabilities. Here, weuse the terms BGP speakers or BGP instances. With the help of these BGPinstances, the autonomous systems exchange messages with each other,either confirming the stored state or giving information about changeswhich should be taken into account in routing. FIG. 1 indicates how thesystem responds to a link failure, under the control of the BGPprotocol. In this case the link between the autonomous systems AS-6 andAS-8 is disrupted. As the response to the malfunction—the response isindicated by the arrows—so-called update messages are propagated throughthe whole network, or the eleven autonomous systems AS-1, . . . , AS-11receive update messages, as applicable, prompting them to recalculateroutes which are optimal in terms of a local metric.

FIG. 2 shows the same network of autonomous systems as in FIG. 1. FIG. 2shows a rapid response in accordance with the invention to the failureof the link between the autonomous systems AS-6 and AS-8. In accordancewith the invention, messages are sent to autonomous systems which lie onsubstitute routes for routes which pass along the failed link. Theautonomous system AS-8 sends messages about the link failure to theautonomous system AS-7, and this in turn to the autonomous system AS-5.Since the autonomous system AS-8 can reach all the autonomous systems inthe right-hand half of the figure, i.e. the autonomous systems AS-1 toAS-4 and AS-6, via the autonomous systems AS-7 and AS-5, the autonomoussystem AS-5 does not need to propagated further the message about thelink failure which it received from AS-8. In an analogous way, theautonomous system AS-6 sends a message to the autonomous system AS-5.The latter then informs the autonomous system AS-7. It is thus theautonomous systems AS-5 to AS-8 which are affected by the link failure,and which provide or identify, as applicable, substitute routes for theroutes which pass along the failed link. In contrast to the conventionalresponse shown in FIG. 1, no messages need to be propagated over thewhole network. In the figure, the autonomous systems AS-1 to AS-4 andAS-9 to AS-11 receive no messages about the link failure, and do notneed to make any modifications.

In the case of a link failure which lasts longer (a persistent error),it is logical to propagate messages through the entire network, tooptimize the routing throughout the network. For this reason provisionis made as shown in FIG. 3 for propagating BGP update messages in thewhole field if the failed link has not recovered after a certain periodof time, for example 10 minutes. The vertical axis in FIG. 3 shows threedifferent phases of the method in accordance with the invention, namelythe phase (Fail) in which the link failure is recognized, the phase(Recv) in which a recovery of the link is signaled if this takes placewithin the time period, and a phase (Pererr) which shows the procedureif the fault concerned is a longer lasting one. On the horizontal axisare shown two BGP speakers or BGP instances, namely BGPspk1, a BGPinstance to which the fault is directly signaled, that is it belongs tothe autonomous system which is adjacent to the failed link, and the BGPinstance BGPspk2, which belongs to an autonomous system which isinformed of the link failure by the BGP instance BGPspk. Three softwareor protocol modules, as applicable, of the first instance BGPspk1 areshown, namely DCT (detection), a module which detects the link failure,the module FSR (fast scope rerouting) which deals with the response inaccordance with the invention or the emission of messages, asapplicable, and BGP, the corresponding BGP protocol software (in thisconnection, one refers also to a BGP routing engine). For the second BGPinstance, the fast scope rerouting module FSR is shown. In this diagram,the time axes run from above to below, i.e. messages or events, asapplicable, which appear further down are later in time. Before thenotification of the link failure, Linkfail, so-called keepalivemessages, BGP (Keepalive), provided by the BGP protocol are forwarded tothe BGP software, BGP, within the first BGP instance, BGPspk1, i.e.orderly functioning of the link is being signaled. After the failure ofthe link, the malfunction is detected, for example by the absence ofkeepalive messages, BGP (Keepalive) (in FIG. 3, the detection of thelink failure corresponds to the Linkfail message). The FSR software isinformed of link failure (the corresponding message in FIG. 3 is called‘notify’). The FSR software in BGP instance 1, BGPspk1, sends a message,FSRlinkdown, to the FSR software in BGP instance 2, BGPspk2, which inturn sends a corresponding message, FSRlinkdown, along a substituteroute or numerous substitute routes, as applicable. The messageFSRlinkdown notifies the receiver instance concerned about the linkfailure, and at the receiver initiates a rapid response to the linkfailure, in accordance with the invention. During the rapid response tothe link failure, regular functioning of the link which has failed issimulated at the BGP protocol level. For this purpose, the FSR softwareFSR in the first BGP instance, BGPspk1, sends BGP keepalive messages,BGP (Keepalive), to the BGP software BGP. The FSR software FSR acts,so-tospeak, as a proxy for the BGP instance at the other end of thefailed link, to block any recalculation of routes in the network by theBGP protocol.

If, before the time period expires, the operability of the link isrestored again, and the first BGP instance BGPspk1 is informed of this(in the diagram this is indicated by the message Linkrecv), the FSRsoftware FSR in the first BGP instance, BGPspk1, uses the messageFSRrecv to inform the second BGP instance, BGPspk2, that the link hasgone back into service. This FSRrecv message is propagated along thesubstitute route. After receiving the message about the failure of thelink, the BGP instances which lie along the substitute route will havereplaced the routes which pass via the failed link by other routes, andwill have identified the substituted routes as temporarily unavailable.The routes identified in this way can be put back into service againafter the message about the recovery of the link is received. After thefailed link has recovered, the first BGP instance, BGPspk1, will againreceive BGP keepalive messages, BGP (Keepalive) via the link which hasgone back into service.

The third period Pererr (standing for ‘persistent error’) shows theresponse in the case of longer-lasting link failures. After a timer Texp(Timer Expired) has expired, FSR software FSR in the first BGP instance,BGPspk1 sends a message FSRpererr (standing for ‘FSR persistent error’)to the second BGP instance BGPspk2, by which it signals that the faultis a longer lasting one. The routes which are marked as temporarilyunavailable can now be removed from the routing table(s). The FSRsoftware FSR of the first BGP speaker, BGPspk1, now no longer simulatesto the BGP software the operability of the link which has failed, butinstead sends a notification BGPlinkdown which informs the BGP softwareBGP about the failure of the link. As the response at this point, BGPupdate messages (Update) are propagated throughout the entire network,and initiate a recalculation of the routes.

Generally, the selection of alternative routes through the routingdomains or BGP instances, as applicable, along the substitute routes ismade by reference to two criteria, namely first that the substituteroute does not pass along the link which has failed and (the substituteroute must satisfy the condition that it represents a REAL substitutefor the failed link) secondly that the substitute route is optimalaccording to some metric which is used locally. In effect, thesubstitute route provides for the routing of data packets a substitutionfor the link which has failed. One metric for determining the bestalternative route when there are several options for determining analternative route could, for example, take into account such criteria asthe number of hops to a destination. The metric used in each case islocal insofar as the routing strategies of other routing domains, whichare not known to the routing domain concerned, are not taken intoconsideration. It is to be recommended, above all in respect of thedifferent routing strategies or policies, that several alternativeroutes are identified or selected, as appropriate, for a route which hasfailed, and several substitute routes are provided. This combination canalso be described as an array of alternative routes or an array ofsubstitute routes. The usefulness of several substitute routes will beexplained in more detail by reference to FIG. 4. This figure showsautonomous systems AS-1 to AS-7. Suppose the link between autonomoussystems AS-1 and AS-4, shown as a dashed line, has failed. From theautonomous system AS-1 to autonomous system AS-4, two substitute routesare now provided, passing respectively through the autonomous systemAS-2 or the autonomous system AS-3. The autonomous systems AS-5, AS-6and AS-7 can be reached via the autonomous system AS-4. Assume that datapackets which are transmitted from the autonomous system AS-1 to theautonomous system AS-7 are not to be transmitted via the autonomoussystem AS-2, for example because on contractual grounds the operator ofautonomous system AS-2 does not forward traffic of this type, or becausehe is of a different nationality so that security considerations meanthat traffic of this type should not be forwarded via his routingdomain. For this case, a second route is available, namely via theautonomous system AS-3, over which traffic can be forwarded to theautonomous system AS-7. The two different routes, via the autonomoussystems AS-2 and AS-3, can be selected depending on the autonomoussystem to which the traffic is to be transmitted, for example AS-5, AS-6and AS-7, and depending on the routing strategy of the destinationnetwork concerned. The provision of several substitute routes can inthis way contribute to taking into account the routing strategy inrouting the traffic along a substitute route.

1. A method for rapidly responding to the failure of a link between tworouting domains in a packet-oriented network, comprising: detecting thefailure of the link by one of the routing domains; and providing atleast one substitute route to a destination point for at least one routeto the destination point, which passes via the failed link, wherein amessage about the link failure is propagated only along the at least onesubstitute route, by means of which routing domains lying along thesubstitute route are notified, and wherein routing domains which havebeen notified and which lie along the at least one substitute routeadjust their inter-domain routing to give a routing to the destinationpoint along the at least one substitute route, until all the routingdomains on the at least one substitute route have adjusted theirinter-domain routing to give a routing to the destination point alongthe at least one substitute route.
 2. The method in accordance withclaim 1, further comprising: notifying a router in a routing domainabout the link failure; selecting an alternative route to the routewhich passes via the failed link, which does not pass via the failedlink by the router; specifying an address of a router in the nextrouting domain on the alternative route as the next destination for theinter-domain routing to the destination point; and sending a message tothe next routing domain on the alternative route, notifying the nextrouting domain about the link failure.
 3. The method in accordance withclaim 1, wherein a router in a routing domain is notified about the linkfailure, wherein for a route which passes via the failed link, therouter checks whether a substitute route has already been set up, andwherein, if there is such a substitute route, no message about the linkfailure will be sent to the next routing domain on the substitute route.4. The method in accordance with claim 2, wherein a router in a routingdomain is notified about the link failure, wherein for a route whichpasses via the failed link, the router checks whether a substitute routehas already been set up, and wherein, if there is such a substituteroute, no message about the link failure will be sent to the nextrouting domain on the substitute route.
 5. The method in accordance withclaim 1, wherein a router in a routing domain is notified about the linkfailure, wherein for each of the routes which pass via the failed link,the router selects alternative routes which do not pass via the failedlink, and wherein the address of a router belonging to the next routingdomain along the alternative route concerned is determined as the nextdestination for the inter-domain routing to the destination point of theroute concerned which has failed.
 6. The method in accordance with claim2, wherein a router in a routing domain is notified about the linkfailure, wherein for each of the routes which pass via the failed link,the router selects alternative routes which do not pass via the failedlink, and wherein the address of a router belonging to the next routingdomain along the alternative route concerned is determined as the nextdestination for the inter-domain routing to the destination point of theroute concerned which has failed.
 7. The method in accordance with claim3, wherein a router in a routing domain is notified about the linkfailure, wherein for each of the routes which pass via the failed link,the router selects alternative routes which do not pass via the failedlink, and wherein the address of a router belonging to the next routingdomain along the alternative route concerned is determined as the nextdestination for the inter-domain routing to the destination point of theroute concerned which has failed.
 8. The method in accordance with claim1, wherein a router selects more than one alternative route to a routewhich passes via the failed link, such that the selected alternativeroutes do not pass via the failed link, and wherein an address of arouter which belongs to the next routing domain on an alternative routeis determined as the next destination for the routing to the destinationpoint of the failed link and for at least one further alternative routethe address of a further router which belongs to the next routing domainon the further alternative route is determined as the alternative nextdestination for the inter-domain routing to the destination point. 9.The method in accordance with claim 2, wherein a router selects morethan one alternative route to a route which passes via the failed link,such that the selected alternative routes do not pass via the failedlink, and wherein an address of a router which belongs to the nextrouting domain on an alternative route is determined as the nextdestination for the routing to the destination point of the failed linkand for at least one further alternative route the address of a furtherrouter which belongs to the next routing domain on the furtheralternative route is determined as the alternative next destination forthe inter-domain routing to the destination point.
 10. The method inaccordance with claim 3, wherein a router selects more than onealternative route to a route which passes via the failed link, such thatthe selected alternative routes do not pass via the failed link, andwherein an address of a router which belongs to the next routing domainon an alternative route is determined as the next destination for therouting to the destination point of the failed link and for at least onefurther alternative route the address of a further router which belongsto the next routing domain on the further alternative route isdetermined as the alternative next destination for the inter-domainrouting to the destination point.
 11. The method in accordance withclaim 5, wherein a router selects more than one alternative route to aroute which passes via the failed link, such that the selectedalternative routes do not pass via the failed link, and wherein anaddress of a router which belongs to the next routing domain on analternative route is determined as the next destination for the routingto the destination point of the failed link and for at least one furtheralternative route the address of a further router which belongs to thenext routing domain on the further alternative route is determined asthe alternative next destination for the inter-domain routing to thedestination point.
 12. The method in accordance with claim 1, wherein arouter selects more than one alternative route to a route which passesvia the failed link, whereby the selected alternative routes do not passvia the failed link, wherein an address of a router which belongs to thenext routing domain on a first alternative route is determined as thenext destination for the routing to the destination point of the routewhich passes via the failed link, and for at least one secondalternative route the address of a router which belongs to the nextrouting domain on the second alternative route is also determined as thenext destination for the inter-domain routing to the destination point,and wherein for inter-domain routing over a substitute route for theroute which passes via the failed link, the next destination isdetermined by reference to parameters which relate to data packets. 13.The method in accordance with claim 2, wherein a router selects morethan one alternative route to a route which passes via the failed link,whereby the selected alternative routes do not pass via the failed link,wherein an address of a router which belongs to the next routing domainon a first alternative route is determined as the next destination forthe routing to the destination point of the route which passes via thefailed link, and for at least one second alternative route the addressof a router which belongs to the next routing domain on the secondalternative route is also determined as the next destination for theinter-domain routing to the destination point, and wherein forinter-domain routing over a substitute route for the route which passesvia the failed link, the next destination is determined by reference toparameters which relate to data packets.
 14. The method in accordancewith claim 1, further comprising: providing a protocol which provides anetwork-wide propagation of messages for determining or calculatingoptimal routes, wherein, after a link failure, any redetermination ofthe optimal routes for inter-domain routing to take into account thelink failure is suppressed for a time period by the protocol.
 15. Themethod in accordance with claim 14, wherein, after the time period hasexpired, a network-wide propagation of messages for the determination ofoptimal routes for inter-domain routing is then undertaken if the linkfailure is still extant.
 16. The method in accordance with claim 14,wherein the protocol used for the redetermination of optimal routes isthe Border Gateway Protocol (BGP) protocol.
 17. The method in accordancewith claim 1, wherein a route which has been replaced by an alternativeroute is marked with respect to its possible reconnection or restorationto service.
 18. A router comprising mechanisms for carrying out themethod of the claim 1.